Hair cells of the inner ear transduce the mechanical energy of sound or head movements into the electrical and chemical signals that are the currency of the nervous system. These signals arise from currents flowing through ion channels of several classes: mechanically-gated ("transduction") channels, voltage-gated channels and ligand-gated channels. It is known that hair cell signals are transformed by modulatory processes within the hair cells, sometimes activated by input from other cells, but underlying mechanisms are poorly understood. In the best-known examples, calcium ions have been implicated as intermediaries (second messenger molecules) in the transformation. This application proposes to investigate the roles of other candidate second messengers: cyclic nucleotides and nitric oxide, in modulating hair cell signals. The sensory epithelium of the mammalian utricle, a vestibular organ, will serve as the test preparation. The cyclic nucleotide experiments are motivated by the discovery in hair cells of cyclic-nucleotide-gated (CNG) channels, which are opened by the binding of cyclic nucleotides. In photoreceptors and olfactory neurons, CNG channels are the target ion channels of the stimulus, i.e., the transduction channels. In hair cells this role is fulfilled by different, mechanically-gated channels. Therefore, it is proposed here that current through the CNG channels serves to modulate, rather than initiate, the hair cell signal. This will be tested by recording hair cell signals while simultaneously stimulating them: mechanically and activating their CNG channels. Nitric oxide (NO) has recently emerged as a modulatory substance within the nervous system and elsewhere. As a short-lived gas that permeates membranes, it has the unusual property that it can influence any targets within a certain volume around its site of generation. Such a messenger could have profound impact within the networks of inner ear sensory epithelia. Preliminary data show that NO can modulate certain voltage- gated channels within hair cells and thereby affect the receptor potential. This application proposes to investigate whether NO is produced within the sensory epithelium, by hair cells or other elements, and whether it modulates hair cell transduction. These experiments may expand our understanding of complex modulatory processes that are likely to occur within hair cell sensory epithelia and to contribute substantially to the normal function of the inner ear.